This invention concerns a phase discriminator and data standardizer that detects the difference in phase between an input data signal and a variable-frequency reference data clock signal and produces an output for changing the frequency of the clock signal to reduce the measured phase difference, while also providing the input data as an output in a standardized format that is synchronized with the reference data clock.
Disk file systems are conventionally used as information storage adjuncts to computer systems. In a disk file storage system, data is recorded by flux reversals placed on the surface of a magnetic disk. The information content of the data lies in the time separating any two flux reversals. To obtain information previously recorded on a disk, the time between two flux reversals must be accurately measured to allow the information to be retrieved. The instantaneous frequency of the data read from a disk varies due to variation in the rotational velocity of the disk. In systems where the data is provided by the disk file system to another apparatus in synchronism with a data clock, means must be provided for varying the frequency of the data clock in a closed loop operation to track any variations in input data speed resulting from changes in the rotational velocity of the disk. Further, the data must be standardized to provide an output that has a predictable time relationship to the reference data clock signal.
In a disk file storage system, the reference data clock is normally produced by a phase-locked loop including a voltage controlled oscillator (VCO) whose frequency is controlled by a phase discriminator that compares the phase relationship of the reference data clock to the phase of the incoming data pulses. Typically, such a phase discriminator, after detecting a phase difference, causes the frequency of the VCO to be varied incrementally to align the phase of the clock and data signals.
The exact placement of magnetic transitions on a disk can be altered in time relative to adjacent transitions. Such positional variations can result from imperfections in the recording medium, flux leakover from adjacent transitions, and noise and imperfections in the detection process. As a result, the digital data pulses derived from the flux transitions will not have a predictable position relative to the reference data clock. Therefore, a data standardizer must make a precise determination of which clock cycle a data pulse occurs in and produce as an output a pulse that occurs in a manner that is predictably standardized in its time relationship to the reference data clock.
Existing phase discriminators employed to detect the difference in phase between a data signal and a reference data clock signal typically employ circuitry that is sensitive to race conditions in that a plurality of circuit transitions must be made in parallel. A variation in the relative delays of these parallel transitions can produce unwanted variations in the phase discrimination and data standardization processes.
As the speeds of data transfer and data processing equipments increase with each new generation of computers, the destabilizing effects of multiple path conditions on phase detection will only grow in magnitude. Reduction of race conditions will enhance the ability of phase detectors to synchronize data obtained from a disk file system with a data clock signal and to standardize the data in a predictable manner.